The present invention relates to an in-situ method and apparatus for end point detection during chemical mechanical polishing, and more particularly to a method and apparatus in which localized areas of the surface of a semiconductor wafer or substrate which is undergoing chemical mechanical polishing are monitored to detect the removal of material from the localized wafer surface areas.
The following literature references describe chemical mechanical polishing and various prior art end point detecting techniques.
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Manufacture of semiconductors has become increasingly complex as the device densities increase. Such high density circuits typically require closely spaced metal interconnect lines and multiple layers of insulating material, such as oxides, formed atop and between the interconnect lines. Surface planarity of the semiconductor wafer or substrate degrades as the layers are deposited. Generally, the surface of a layer will have a topography that conforms to the sublayer, and as the number of layers increase the non-planarity of the surface becomes more pronounced.
To address the problem, chemical mechanical polishing (CMP) processes are employed. The CMP process removes material from the surface of the wafer to provide a substantially planar surface. More recently, the CMP process is also used to fabricate the interconnecting lines. For example, when depositing copper leads or interconnect lines, a full layer of the metal 13 is deposited on the surface of the wafer 10 having grooves 12 formed in an oxide layer 11 as shown in FIGS. 1A and 1B. The metal layer 13 may be deposited by sputtering or vapor deposition or by any other suitable conventional technique. The oxide layer, such as doped or undoped silicon dioxide, is usually formed by chemical vapor deposition (CVD). The metal layer covers the entire surface of the wafer and extends into the grooves. Thereafter, individual leads 16 are defined by removing the metal layer from the surface of the oxide. The CMP process may be used to remove the surface metal leaving the leads 16 in the grooves. The leads are insulated from one another by the intervening oxide layer.
In general, to carry out the CMP process, a chemical mechanical polishing (CMP) machines is used. Many types of CMP machines are used in the semiconductor industry. CMP machines typically employ a rotating polishing platen having a polishing pad thereon, and a smaller diameter rotating wafer carrier which carries the wafer whose surface is to be planarized and/or polished. The surface of the rotating wafer is held or urged against the rotating polishing pad. A slurry is fed to the surface of the polishing pad during polishing of the wafer.
It is desirable to precisely determine when the material has been removed from the upper surface of the wafer during the CMP process. This not only prevents discarding of over-polished wafers, but also minimizes the necessity of re-polishing any under-polished wafers. There are many possible ways of determining when to stop the CMP process. Typical methods include: (1) detecting frictional change as the top layer of metal is polished away to expose the silicon oxide layer by monitoring the current to the platen and carrier motors, and (2) monitoring thermal and acoustic signatures from the polishing pad. Electrical impedance, conductance and capacitance can also be used to determine the presence of the metal layers.
More recently, optical measurement has been used in the art with the CMP process. For example, U.S. Pat. No. 5,838,448 uses interferometry and describes detecting the thickness of a thin layer, or the changes in the film thickness, by measuring reflectance variations caused by a change in the incidence angle of incident light. U.S. Pat. No. 5,835,225 describes using reflectance measurements to determine a particular surface property of the substrate. U.S. Pat. No. 5,433,651 describes a method and apparatus for viewing the wafer during polishing and end-pointing the CMP process when a prescribed change in the in-situ reflectance corresponds to a prescribed condition of the polishing process.
While these techniques have provided improvements to the CMP process, these methods provide average (global) characteristics of the whole wafer surface, rather than those of smaller, localized regions or areas of the wafer. This means that, although one part of the wafer may get polished before another, the global system is not typically able to differentiate between over-polished and under-polished regions of the wafer.
In another prior art technique, as described in U.S. Pat. No. 5,972,787, indicator areas are provided on the wafer. These indicator areas are formed of blocks of parallel metal lines with varying line widths and pattern factors that are chosen to violate existing ground rules in such a way that they will be dished out using the standard consumable set (pad/slurry) of a given metal CMP process. The blocks are then inspected to determine the extent of polishing. While this technique provides for indicating the polishing in certain areas of the wafer, the process requires that the CMP step be interrupted for the inspection to take place. Further, the indicator areas require formation of the blocks which add an additional step to the already complex fabrication process.
In addition, the copper (Cu) damascene process is emerging as a critical technology to produce high-speed, high-performance, and low energy-consuming Ultra-Large-Scale Integrated (ULSI) circuits. In copper damascene, the CMP process is employed to remove the excess copper and barrier materials (typically Ta, Ti, TaN or TiN) and to form interconnects inside the trenches in the inter-layer dielectric (ILD, typically SiO2 or polymers). The copper damascene process adds additional complexities to the CMP process. It has been reported that the material removal rate of Cu strongly depends on the pattern geometry. The nonuniform pattern layout usually causes nonuniform polishing across the die area, and results in partial overpolishing on the area with higher Cu fraction and dishing on the soft Cu lines. The Cu loss and surface nonuniformity due to overpolishing and dishing may affect the reliability of interconnects and must be minimized. Additionally, the nonuniformity of initial Cu coating, the spatial variation of the process parameters (velocity, pressure, slurry transport, etc.), and the process random variation will increase the within-wafer and within-lot nonuniformity of polishing. These result in a variation of the completion time, or the endpoint, of the Cu CMP and impact the process yield. In order to reduce the variance of polishing outputs (uniformity, overpolishing and dishing), it is desirable to integrate an in-situ sensing and endpoint detection technique with the process optimization schemes to improve process performance.
The wafer-level endpoint for the copper CMP process may be defined as the time when the excess Cu and barrier layers are fully cleared up on a specified number (or percentage) of dies of a wafer. Due to the polishing nonuniformity, all the dies on a wafer generally will not reach the endpoint at the same moment, and some of the dies may be overpolished. Thus the endpoint of CMP can be a representation of the optimal polishing time at which the number of out-of-specs dies (either under- or over-polished) reaches a minimum and the process yield is maximized. However, the remaining Cu thickness on each die area is difficult to measure in real-time to determine the endpoint. Most of the prior art in-situ sensing techniques rely on indirect methods to detect the moment of Cu/barrier clear-up, such as the changes in the friction force, the ion concentrations of the Cu/barrier materials, and the electrical impedance on the surface. However, these methods are limited due to the lack of reliability and the problem of high noise-to-signal ratio in practical applications. Moreover, all these techniques provide only average information over a relative large area (usually wafer-level) and lack the capability of sensing within-wafer and die-level uniformity. Therefore, these methods can just be used as supplementary methods with other primary metrology to assure the detection of endpoint.
Recently, the capability of a photoacoustic technique on the thickness measurement of multi-layer stacked films has been investigated. Two optical excitation pulses are overlapped on the surface of the coating to form an interference pattern. Absorption of light by the film generates counter-propagating acoustic wave. By measuring the acoustic frequency, the film thickness can be calculated. However, this method is limited to a blanket area with the dimensions much larger than the beam size. It is difficult to model the generation and the propagation of the acoustic wave in thin Cu film on the patterned area. Hence, this method is currently limited to the measurements for blanket wafers or large patterns which can be simulated as blanket areas.
Among all the endpoint detection techniques, optical sensing techniques may prove to be the most successful. Interferometry technology is employed to measure the film thickness based on the interference of light from the surface of the top and the underlying layers. This may be suitable for measuring transparent films such as dielectric layers, but not effective for opaque metal films. In theory, the reflectance measurement may be used for detecting the surface topography and the metal area fraction on the surface. Moreover, because the reflectance of patterned surface is influenced by the topography of the pattern, it may also be possible to gain information on surface planarity and dishing by this metrology. While the reflectance technique holds promise, significant development is needed to provide a practical end point detection system and method.
Accordingly, there is a need for an improved method and apparatus that can continuously, and in-situ, monitor localized regions of the wafer surface during the CMP process.
It is an object of the present invention to provide an in-situ method and apparatus for monitoring localized regions of the wafer surface during the CMP process.
It is another object of the present invention to provide a method and apparatus which continuously monitors the polishing progress at different areas of the wafer, and may also be used to determine the end point for removal of material from the surface of the wafer.
It is a further object of the present invention to provide a method and apparatus which employs the difference in reflectance between different materials on a wafer to monitor the polishing progress and/or end point at selected regions on the wafer surface.
It is yet another object of the present invention to provide a method and apparatus which monitors reflectance at various surface areas of the wafer and controls the polishing process at said areas to achieve substantially uniform removal of metal during polishing.
It is an even further object of the present invention to provide an in-situ method and apparatus for monitoring surface conditions and detecting the process endpoint for cooper damascene CMP.
The foregoing and other objects of the invention are achieved by a chemical mechanical polishing method and apparatus in which a rotating polishing platen and polishing pad of a first diameter polishes a wafer carried by a wafer carrier. A window is formed in the polishing platen and pad whereby said window periodically scans across the underside of the wafer. An optical detector, such as a fiber optic cable, transmits light through the window onto the surface of the carrier and receives light reflectance through the window from said wafer surface as it rotates past the window and means are provided for monitoring the reflected light, and for controlling the polishing process at localized regions of the wafer responsive to the reflected light information.
More specifically, the chemical mechanical polishing method and apparatus includes a wafer carrier that has a membrane having a central and concentric pressure chambers or compartments which define corresponding zones or regions on the wafer surface. An actuator is provided to control the pressure applied to the central and concentric compartments and thereby control the rate of removal of material from the wafer surface at each of the corresponding zones, and the actuator is engaged responsive to reflected light received at each of the zones.
In another aspect of the present invention, a method of chemical mechanical polishing is provided comprising the steps of: providing a CMP machine which includes a polishing pad and a wafer carrier having multiple chambers that allow for independently varying pressure within the chambers that urge against a wafer at corresponding localized regions on the wafer; measuring the reflectance of the surface of the wafer during polishing at each of the localized regions on the wafer; processing the reflectance data to determine the state of polishing within each of the localized regions; and independently adjusting the pressure within any one of the chambers responsive to the state of polishing within each of the corresponding localized regions.